1. Field of the Invention
The invention relates generally to three-phase electric motors, and more particularly to systems and methods for insulating the Y-points of three-phase electric motors in which the Y-points are not grounded or tied to a reference voltage, and may consequently experience high voltages.
2. Related Art
Three-phase electric motors are commonly used in many different applications. One type of system in which three-phase electric motors are predominantly used is an electric submersible pump (ESP) system. ESP's are used in the oil and gas production industries to produce these fluids from wells. Three-phase motors can be designed to fit within the narrow confines of a well, yet still produce the substantial amount of lifting power required to pump the fluids from the wells, which may be thousands of feet deep.
Three-phase electric motors typically use either a “Y” configuration or a “delta” configuration. These motors may also be referred to as “Y-wound” or “delta-wound” motors. A three-phase motor has three coils (or sets of coils) which generate the magnetic fields that drive the motor. In a “Y” configuration, power is applied to one end of each coil (each coil receiving a different phase of the three-phase power), and the second end of each coil is tied to the others at a junction that is referred to as the Y-point. The term “Y-point” refers to the fact that the three conductors from the coils form a “Y” at the junction. The Y-point is also sometimes referred to as the “Wye”-point. In a delta configuration, rather than having the second end of each coil tied together at a single junction, the first end of each coil is tied to the second end of one of the other coils. This can be represented diagrammatically by a triangle, with each side of the triangle representing one of the coils. In a delta configuration, power is applied to the corners of this triangle.
The three-phase electric motors that are employed in ESP systems predominantly use the Y-configuration. Three-phase power is supplied to the motor through a cable that runs from a power source at the surface of the well, through the well to the motor of the ESP system. Each phase is applied to a first end of one of the coils of the motor. As noted above, the second end each of the coils is tied to the others at the Y-point. The Y-point is not connected to anything else, and is intended to be electrically isolated from anything else. The Y-point is therefore electrically insulated to ensure that it is isolated. Because the Y-point is a junction of three conductors, however, it is difficult to insulate the Y-point well using conventional techniques.
During normal operation, the voltage at the Y-point is very low. In an ideal system, the sum of the voltages of each of the three phases would be zero. When the voltage at the Y-point is low, there is little electrical stress on the insulation of the Y-point. There are instances, however, when the voltage of the Y-point becomes very high, and the insulation of the Y-point can fail. For instance, one of the conductors of the cable or motor can become electrically connected to ground. This can occur in the motor laminations, shaft, or housing. (Another type of “short” would be an energized-conductor-to-energized-conductor fault, which this invention does not address.) The motor can continue to operate in this condition, but as a result, the voltage at the Y-point (which is the sum of the three phases) increases substantially. If the increased voltage at the Y-point causes it to short out as well, the motor will fail. Because of the expense associated with retrieving the failed motor from the well to replace it, this is a very serious problem.
It would therefore be desirable to provide systems and methods for insulating the Y-points of three-phase electric motors so that they can withstand voltages up to or exceeding the motor nameplate voltage, particularly in applications such as ESP systems.